/Engine/Source/Editor/EditorFramework/Public/Toolkits/ToolkitManager.h
Called by a toolkit host itself right before it goes away, so that we can make sure the toolkits are destroyed too
void OnToolkitHostDestroyed ( [IToolkitHost](API\Editor\EditorFramework\Toolkits\IToolkitHost) * HostBeingDestroyed )